In the modern illuminating devices, especially in the LED illuminating devices, an improved solution having a dimming function is put forward in order to obtain luminance adapted to different environments. Particularly, this function is mainly realized by a TRIAC phase-cut dimmer. For instance, in a common dimming system for an LED illuminating device as shown in FIG. 1, the LED will be caused to flicker when the TRIAC phase-cut dimmer is connected to an LED driving circuit, then an additional damping circuit is usually needed for inhibiting the flickering. In such a damping circuit, a main current flows through a high-power damping resistor R12 (for instance, 150 R/2 W herein) having a big resistance value, and subsequently flows into a dimmer for controlling the LED luminance. As the damping resistor R12 has consumed a lot of power, the power of the LED driving circuit is reduced by 3%-5%. In addition, the service lifetime of electronic elements of the circuit also will be adversely affected after the electric energy consumed on the damping resistor R12 is converted into heat energy.
In order to improve the power of the LED driver, the power lost on the high-power damping resistor should be reduced; as a result, a dynamic switch is often used. The disadvantage of such method lies in that the dynamic switch used cannot recognize whether the driver is connected with a dimmer, i.e., an operation state of the dimmer cannot be recognized, which thus results in that, when a driving circuit of an illuminating device is not connected with the dimmer or the dimmer has been adjusted to a predetermined dimming level, R12 is still in an operation state and further affects an output power of the driving circuit of the illuminating device.